I'm not sure that this is strictly true. Yes there is a way to get exempted from the "No hire & reward" rule for a Permit aircraft in order to train on it, but I think you'll find that you have to have a very good reason for the CAA to grant an exemption i.e either having bought an aircraft and then require training on it by a third party or about to buy a share in said aircraft. In the last case, I am reliably informed that the CAA grants a 'one flight only' exemption. Guess how I know?
And I can reverse it, and say "Guess how I know". I "instruct" people on the JP who are not owners - we do it under the AIC I mentioned above. The owners set a limitation that they are not to fly solo wihtout buying a share - but the CAA / AIC set no such restriction.
How much would it realistically cost to take a low hour PPL holder to solo in the JP.
See
Swords Aviation link, which details costs both as an owner, and under the AIC. I believe the insurance requires a total of 125hours+ prior solo... As a wild guess, for an able PPL, who is willing to study, work on 5-10 hours to solo. However, the membership / CAA CAP 632 for Mil Aircraft is not really designed around "getting solo", but completing a course to a Full Exemption which allows you to fly Pax. Getting solo would only be ~half, or even less, of the syllabus...